The metal-all-dielectric optical filter in the physics package of a lamp-pumped rubidium atomic clock was investigated as a compact passive filter to condition the light emitted by the rubidium lamp. The use of such a design could enable spectral conditioning of devices using natural rubidium and reduce reliance on isotope-enriched 85 Rb vapor cells. In the case of lamp-pumped atomic clocks, the broad spectrum due to the presence of different noble gases in the rubidium lamp and their distinct emission lines in the lamp radiation could cause an increase in the optical background, a decrease in double-resonance contrast, additional optical pumping, and degradation of the microwave–optical double-resonance discriminator signal. To solve the problem described above, a one-dimensional defective metal–dielectric photonic crystal filter was designed, manufactured, and used in a rubidium atomic clock physics package. The simulation results obtained via the transfer matrix method revealed that the proposed structure can generate a wavelength-selective defect-mode passband within the photonic band gap, with efficient electric-field localization and group delay in the main defect-mode region. Conversely, our experimental results with a defective photonic crystal filter and a 600 nm long-pass optical filter, indicated that the photonic crystal filter provided higher wavelength-scale spectral selectivity, suppressed a significant fraction of the unwanted broadband lamp background, and preferentially transmitted the spectral region containing the rubidium D-line emissions. This would yield a suitable double-resonance signal for error-signal extraction compared with the case where the long-pass filter was used. The photonic-crystal thin-film filters increased the peak-to-peak double-resonance amplitude by approximately 11.3 times. They increased the zero-crossing frequency-discrimination slope by approximately 11.8 compared with that of the 600 nm long-pass filter. The equivalent RMS frequency noise of the open-loop discriminator signal decreased from about 95 Hz to about 17 Hz. However, this does not imply that thin-film photonic crystal filters can fully replace conventional rubidium atomic filter cells. Rather, this aim explores a cost-reduction approach using a natural-abundance rubidium cell and reduces the need for additional enriched 85 Rb vapor filter cells. These results suggest that photonic-crystal thin-film filters can be useful passive spectral-conditioning elements for compact lamp-pumped rubidium atomic-clock physics packages.
The metal-all-dielectric optical filter in the physics package of a lamp-pumped rubidium atomic clock was investigated as a compact passive filter to condition the light emitted by the rubidium lamp. The use of such a design could enable spectral conditioning of devices using natural rubidium and reduce reliance on isotope-enriched 85 Rb vapor cells. In the case of lamp-pumped atomic clocks, the broad spectrum due to the presence of different noble gases in the rubidium lamp and their distinct emission lines in the lamp radiation could cause an increase in the optical background, a decrease in double-resonance contrast, additional optical pumping, and degradation of the microwave–optical double-resonance discriminator signal. To solve the problem described above, a one-dimensional defective metal–dielectric photonic crystal filter was designed, manufactured, and used in a rubidium atomic clock physics package. The simulation results obtained via the transfer matrix method revealed that the proposed structure can generate a wavelength-selective defect-mode passband within the photonic band gap, with efficient electric-field localization and group delay in the main defect-mode region. Conversely, our experimental results with a defective photonic crystal filter and a 600 nm long-pass optical filter, indicated that the photonic crystal filter provided higher wavelength-scale spectral selectivity, suppressed a significant fraction of the unwanted broadband lamp background, and preferentially transmitted the spectral region containing the rubidium D-line emissions. This would yield a suitable double-resonance signal for error-signal extraction compared with the case where the long-pass filter was used. The photonic-crystal thin-film filters increased the peak-to-peak double-resonance amplitude by approximately 11.3 times. They increased the zero-crossing frequency-discrimination slope by approximately 11.8 compared with that of the 600 nm long-pass filter. The equivalent RMS frequency noise of the open-loop discriminator signal decreased from about 95 Hz to about 17 Hz. However, this does not imply that thin-film photonic crystal filters can fully replace conventional rubidium atomic filter cells. Rather, this aim explores a cost-reduction approach using a natural-abundance rubidium cell and reduces the need for additional enriched 85 Rb vapor filter cells. These results suggest that photonic-crystal thin-film filters can be useful passive spectral-conditioning elements for compact lamp-pumped rubidium atomic-clock physics packages.
The optimisation of a double resonance optically pumped (DROP) rubidium (Rb) atomic clock using a reconfigurable physics package testbed is reported. The testbed can incorporate numerous sizes and variations of the following components: optical sources, Rb vapour cells and microwave cavities. DROP measurements were carried out for a range of different optical powers and temperatures. For one of the Rb vapour cells studied, near optimum physics package conditions were achieved at 75 °C with a DROP signal contrast of 8.7%, a linewidth of 2.3 kHz, and an optical power of 25 In addition, the extent of helium permeation through the walls of a commercial Rb integrated cell was determined by immersing a Rb atomic clock in pure helium at atmospheric pressure and monitoring the drift of the 10 MHz output. A fractional drift rate in normal air (with 5 ppm helium) was determined to be nearly 10-11/month.
This thesis presents experimental studies on continuous-wave (CW) laser-microwave double-resonance (DR) spectroscopy and metrology in rubidium (87Rb) vapor cells in view of new high-performance, compact Rb-cell atomic clocks. The Rb vapor cell is confined inside a magnetron-type cavity microwave resonator (MWR). The CW DR spectroscopy involves two resonant electromagnetic fields that are operated simultaneously to interrogate the atoms - the optical field to polarize the atoms by optical pumping, and the microwave field to drive the ground-state hyperfine clock transition that serves as an atomic frequency reference. Details on characterization of compact laser heads and microwave synthesizers used in this work are presented. The vapor cell standards are useful in our everyday lives for applications ranging from telecommunications, navigation, metrology etc. In view of improving the performances of the Rb cell standards, two different clock approaches were studied in detail: The first one is on use of vapor cells (1.4 cm3) whose inner walls are coated with anti-relaxation material. A first ever wall-coated cell clock was demonstrated with good short-term frequency stability. The medium- to long-term stability was found to be limited by the temperature coefficient (TC) of the coating material itself. This work gave the insight to important features to be considered in future anti-relaxation coating materials for atomic standards. The second novel approach involves using a bigger cell (12 cm3) within a newly developed, improved MWR towards a high-performance atomic standard. Adopting a larger cell gives a higher atomic Q-factor signal that improves the short-term clock stability. In this approach, the cell was filled with 87Rb and buffer gases. With this clock, we demonstrate the state-of-the-art short-term stability of <1.4×10−13 τ−1/2. Metrological quantitative measurements on parameters influencing the medium- to long-term stability were studied in view of next generation satellite navigation systems that demand a stability level of <1×10−14 at 10 4 s (equivalent to <1 ns/day). The potential of short-term stability and understanding of the limiting factors on medium- to long-term time scales demonstrated in this study pave the way for future work towards the commercialization of high-performance Rb atomic clocks for a variety of applications.
We report our investigations on a compact high-performance rubidium (Rb) vapourcell clock based on microwave-optical double-resonance (DR).These studies are done in both DR continuous-wave (CW) and Ramsey schemes using the same Physics Package (PP), with the same Rb vapour cell and a magnetron-type cavity with only 45 cm 3 external volume.In the CW-DR scheme, we demonstrate a DR signal with a contrast of 26% and a linewidth of 334 Hz; in Ramsey-DR mode Ramsey signals with higher contrast up to 35% and a linewidth of 160 Hz have been demonstrated.Short-term stabilities of 1.4×10 -13 τ -1/2 and 2.4×10 -13 τ -1/2 are measured for CW-DR and Ramsey-DR schemes, respectively.In the Ramsey-DR operation, thanks to the separation of light and microwave interactions in time, the light-shift effect has been suppressed which allows improving the long-term clock stability as compared to CW-DR operation.Implementations in miniature atomic clocks are considered. IntroductionSince the first invention of lamp-pumped rubidium (Rb) clocks in the 1960's [1,2], the demand for a stable, reliable, and compact Rb frequency standard with low power consumption has been growing, for various applications fields such as industry, telecommunication, navigation, space applications (e.g.GPS [3], GALILEO [4] and COMPASS [5]).Recently, thanks to the semiconductor laser technology, laser-pumped Rb clocks have been considered in several studies, for their better optical pumping efficiency results in short-term frequency stabilities at least one order of magnitude better than their lamp-pumped counterparts [6][7][8][9].This opened up also several new schemes for vapour-cell atomic clocks, such as the continuous-wave (CW) Rb clock [9], coherent population trapping (CPT) Cs clock [10] and pulsed optical pumping (POP) Rb clock [11].In this paper we report our studies on high-performance Rb frequency standards based on the laser-microwave double-resonance (DR) scheme, using both CW and Ramsey interrogation.Both CW-DR and Ramsey-DR schemes require two resonant electromagnetic fields of light and microwave.In the case of CW-DR, the laser optically pumps the vapour of the Rb atoms by creating a polarization of the atomic ground-state; simultaneously the microwave field, generated from a quartz oscillator and is near-resonant with the 87 Rb hyperfine clock transition (energy levels 5 2 S 1/2 │F g =1, m F =0˃ ↔│F g =2, m F =0˃), is applied.The opacity of the Rb vapour to the transmitted light signal as a function of microwave frequency is a measure of the atomic ground state polarization known as CW-DR signal [9].In CW-DR, the steps of optical pumping, microwave interrogation and optical detection are applied simultaneously (figure 1(b)).In the case of Ramsey-DR, these three sequences are separated in time, see figure 1(c): first, during optical pumping a strong laser pulse creates a significant atom population imbalance between
Experiments were carried out to evaluate the performances of a semiconductor laser pumped rubidium (87Rb) atomic clock. Two kinds of Rb gas cells were used and their performances were compared [gas cell A (natural rubidium (87Rb/85Rb =\frac{3}{7}) and buffer gases) and gas cell B (87Rb and buffer gases)]. The highest microwave frequency stabilities were estimated as3.4 \times 10^{-12} \tau^{-1/2}and2.7 \times 10^{-12} \tau^{-1/2}at the optimal gas cell temperatures of 60°C and 48°C for the gas cellsAandB, respectively (τ: integration time). The light shift, i.e., microwave frequency shift induced by laser light, was measured as -0.50 Hz/MHz and -0.11 Hz/MHz for the gas cellsAandBat their optimal operating conditions given above. As an improved experiment by utilizing high temporal coherence of the laser, a novel double resonance spectral line shape with a drastically narrower linewidth was demonstrated. A technique, similar to FM laser spectroscopy, was employed for this purpose by utilizing laser FM sidebands which are induced by microwave frequency modulation and nonlinear susceptibility of three-level87Rb atoms. The minimum linewidth obtained was 20 Hz, which can be used as a sensitive frequency discriminator for an improved87Rb atomic clock.
This thesis presents experimental studies on continuous-wave (CW) laser-microwave double-resonance (DR) spectroscopy and metrology in rubidium (87Rb) vapor cells in view of new high-performance, compact Rb-cell atomic clocks. The Rb vapor cell is confined inside a magnetron-type cavity microwave resonator (MWR). The CW DR spectroscopy involves two resonant electromagnetic fields that are operated simultaneously to interrogate the atoms - the optical field to polarize the atoms by optical pumping, and the microwave field to drive the ground-state hyperfine clock transition that serves as an atomic frequency reference. Details on characterization of compact laser heads and microwave synthesizers used in this work are presented. The vapor cell standards are useful in our everyday lives for applications ranging from telecommunications, navigation, metrology etc. In view of improving the performances of the Rb cell standards, two different clock approaches were studied in detail: The first one is on use of vapor cells (1.4 cm3) whose inner walls are coated with anti-relaxation material. A first ever wall-coated cell clock was demonstrated with good short-term frequency stability. The medium- to long-term stability was found to be limited by the temperature coefficient (TC) of the coating material itself. This work gave the insight to important features to be considered in future anti-relaxation coating materials for atomic standards. The second novel approach involves using a bigger cell (12 cm3) within a newly developed, improved MWR towards a high-performance atomic standard. Adopting a larger cell gives a higher atomic Q-factor signal that improves the short-term clock stability. In this approach, the cell was filled with 87Rb and buffer gases. With this clock, we demonstrate the state-of-the-art short-term stability of <1.4Ã10â13 τâ1/2. Metrological quantitative measurements on parameters influencing the medium- to long-term stability were studied in view of next generation satellite navigation systems that demand a stability level of <1Ã10â14 at 10 4 s (equivalent to <1 ns/day). The potential of short-term stability and understanding of the limiting factors on medium- to long-term time scales demonstrated in this study pave the way for future work towards the commercialization of high-performance Rb atomic clocks for a variety of applications.
The paper presents the realisation of a high-performance vapour-cell atomic frequency standard demonstrator based on laser optical pumping. The development of such devices aims to fully exploit the advantages of using a narrow-band diode laser pump light source instead of the discharge lamps commonly used in Rb gas-cell clocks. These advantages include improved control over the pump light spectrum, higher selectivity of the optical pumping, and increased signal contrast, and thus show the potential for the realisation of Rb atomic clocks with improved short-term frequency stability.